Three pathways have been described by which single-stranded circular phage DNAs can be converted to double-stranded DNA in reactions catalyzed by purified E. coli proteins. The three pathways use different mechanisms of priming the template DNA but share a common mechanism of DNA elongation catalyzed by DNA polymerase III, dnaZ protein and DNA elongation factors I and III. In the simplest pathway fd(or M13) DNA synthesis by the DNA elongation components is primed by RNA synthesized by RNA polymerase. In the most complex pathway, phi X174 DNA synthesis requires dnaB, C(D), DNA binding protein, at least three other E. coli proteins (DNA replication factors X, Y and Z) and the same four DNA elongation proteins. A third pathway used G4(or ST-1) DNA, dnaG protein, and DNA binding protein as well as the DNA elongation proteins. The functions of these purified proteins and the mechanism by which they catalyze DNA synthesis are being studied. BIBLIOGRAPHIC REFERENCES: Wickner, S., Mechanism of DNA elongation catalyzed by E. coli DNA polymerase III, dnaZ protein and DNA elongation factors I and III. Proc. Natl. Acad. Sci. 73: 3511-3515, 1976. Wickner, S.: DNA or RNA priming of G4 DNA synthesis by E. coli dnaG protein. Proc. Natl. Acad. Sci. (in press) 1977.